syscall: fix the deprecated way of parsing routing message on openbsd

OpenBSD 5.5 changed its kernel ABI and OpenBSD 5.6 enabled it.
This CL works on both 5.5 and 5.6.

Fixes #9102.

Change-Id: I4a295be9ab8acbc99e550d8cb7e8f8dacf3a03c5
Reviewed-on: https://go-review.googlesource.com/1932
Reviewed-by: Ian Lance Taylor <iant@golang.org>